state: integrate relation membership with life

state: integrate relation membership with life

Please see https://codereview.appspot.com/6678046/ for relevant discussion
that went in the wrong place dure to my forgetting this prereq.

* scope entry is now handled by running a series of transactions, each
  with asserts corresponding to a valid initial state and changes transforming
  that state into the desired one, until one passes
* scope exit leaves the settings untouched, but will (if necessary) delete
  the scope doc and decrement the refcount; it doesn't care about life.
* a relation can become Dying regardless of its refcount.
* a relation cannot become Dead while its refount is non-zero.
* when a relation is removed, which cannot occur until it is Dead, a cleanup
  document is added to facilitate subsequent removal of unused settings
  documents.

https://code.launchpad.net/~fwereade/juju-core/relation-lifecycles/+merge/129540

(do not edit description out of merge proposal)

[fwereade, 2012-10-13 00:29:44]

Please take a look.

[fwereade, 2012-10-14 19:44:07]

A thought:

https://codereview.appspot.com/6687043/diff/1/state/state.go
File state/state.go (right):

https://codereview.appspot.com/6687043/diff/1/state/state.go#newcode495
state/state.go:495: if err := r.st.settings.Find(sel).All(&docs); err != nil {
I am nervous about this -- N=100000 -- but AIUI it's not OK to modify txn
collections other than via txn. It seems like a RemoveAll would be just the
ticket, were it not for the txn restrictions...

I have a vague hope that we might be able to get away with it -- on the basis
that nothing should ever be watching or otherwise using the settings of a dead
Relation -- but I'm not sure about this, and would appreciate guidance.

[niemeyer, 2012-10-15 15:15:10]

We've discussed a slightly different approach on IRC.

[fwereade, 2012-10-16 10:27:30]

Please take a look.

[fwereade, 2012-10-16 13:29:22]

I'm rejecting this; a followup that wasn't properly -req'd has already been
reviewed, addressing everything there.

https://codereview.appspot.com/6687043/diff/6001/state/state.go
File state/state.go (left):

https://codereview.appspot.com/6687043/diff/6001/state/state.go#oldcode488
state/state.go:488: Assert: D{{"life", Dead}},
Bah, I need better ErrAborted handling below :/.